AGU journal highlights - 13 December 2005

I. Highlights, including authors and their institutions

The following highlights summarize research papers in Geophysical Research Letters (GL). The papers related to these Highlights are printed in the next paper issue of the journal following their electronic publication.

You may read the scientific abstract for any of these papers by going to http://www.agu.org/pubs/search_options.shtml and inserting into the search engine the portion of the doi (digital object identifier) following 10.1029/ (e.g., 2005GL987654). The doi is found at the end of each Highlight, below. To obtain the full text of the research paper, see Part II.

1. Estimating Hurricane-Force Winds

Spaceborne microwave radars measure the velocities of moderate winds near the surface of the ocean by analyzing surface roughness, which increases with wind speed and influences spectral density. However, it is difficult to estimate wind velocities during hurricanes, due to breaking waves, surface foam, and the complex structure of the long wave field (cross seas) generated when the hurricane was at a different location. To better measure hurricane-force winds, Horstmann et al. studied various types of synthetic aperture radar (SAR) wind retrieval schemes applied to the high winds observed in 2004 during Hurricane Ivan. They found that the newly-developed Cmod5 empirical geophysical model function (GMF) outperforms the commonly used Cmod4 GMF in analyzing these high winds. They suggest that continued analysis of SAR wind mapping under extreme wind conditions can be a useful tool for hurricane tracking and prediction.

Title: Can synthetic aperture radars be used to estimate hurricane force winds?

Authors:
J. Horstmann: Institute for Coastal Research, GKSS Research Center, Geesthacht, Germany; Center for Southeastern Tropical Advanced Remote Sensing, University of Miami, Miami, Florida, USA;
D. R. Thompson and F. Monaldo: Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland, USA;
S. Iris: Canadian Space Agency, Longueuil, Quebec, Canada;
H.C. Graber: Center for Southeastern Tropical Advanced Remote Sensing, University of Miami, Miami, Florida, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL023992, 2005

2. Weakened thermohaline circulation will increase snow cover over Europe

Global climate models and paleo-observations have associated periods of weak or absent global thermohaline circulation (THC) with widespread cooling around the North Atlantic. However, the low spatial resolutions of past climate models prevented detailed analyses of the weakened circulation's effects on specific locations in Europe. To study this, Jacob et al. used a high resolution regional climate model nested into a global climate model to perform a THC slowdown experiment, imposing additional freshwater to the northern Atlantic, corresponding to one-sixth of the Greenland ice sheet melting over 100 years. According to the model hierarchy, this reduced the strength of the THC by half, changing atmospheric circulation and enhancing maritime climate influences over Europe. Their model system predicted more than 3 degrees Celsius [5 degrees Fahrenheit] of surface cooling over the North Atlantic, causing winter temperatures in northern Europe to drop by 1.5 degrees Celsius [2.7 degrees Fahrenheit] or more. Precipitation also decreased in their model, especially in summer. The remaining precipitation manifested as snow, increasing the albedo (surface reflectivity) of Scandinavia, northern Germany and Britain, and of mountainous areas in central Europe. If these scenarios occur, the authors say, energy consumption in Europe would likely increase, due to humans responding to such cold temperatures.

Title: Slowdown of the thermohaline circulation causes enhance maritime climate influence and snow cover over Europe

Authors:
Daniela Jacob, Holger Goettel, Johann Jungclaus, Michael Muskulus, Ralf Podzun, and Jochem Marotzke: Max Planck Institute for Meteorology, Hamburg, Germany.

Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL023286, 2005

3. Arctic Ocean change heralds North Atlantic freshening

The global thermohaline circulation (THC) is driven by the formation of deep water in the North Atlantic. Model simulations have shown that freshwater storage and outflow from the Arctic Ocean can potentially affect sensitive areas of deep water formation in the Greenland, Labrador, and Irminger seas, influencing thermohaline circulation intensity. For example, during the late 1960s and early 1970s, freshening of the North Atlantic occurred through large sea ice exports that migrated from icecaps into less frigid latitudes. Similar mechanisms were thought to have driven the formation of low salinity water during the 1980s and 1990s. In contrast to these earliers studies, Karcher et al. determined that the salinity anomaly of the 1990s was a consequence of a large release of liquid freshwater from the Arctic. Using a numerical model and observations, they show that this freshwater release was a result of a change of the Arctic Ocean's thermohaline structure, in response to the very intense North Atlantic Oscillation during that time. Based on their findings, they posit that that a strong link exists between large-scale Arctic Ocean changes and freshwater fluxes to subpolar waters.

Title: Arctic Ocean change heralds North Atlantic freshening

Authors:
M. Karcher, R. Gerdes, F. Kauker, and C. Koberle: Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany;
M. Karcher and F. Kauker: O. A. Sys - Ocean Atmosphere Systems, Hamburg, Germany;
I. Yashayaev: Ocean Circulation Section, Ocean Sciences Division, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada.

Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL023861, 2005

4. Seismic tomography reveals magma intruding within Mt. Etna

On 12 July 2001, an intense seismic swarm was recorded in the upper southern flank of Mt. Etna, accompanying the opening of surface fractures which led to the 17 July-9 August flank eruption. Past studies indicated that most such eruptions have been preceded by changes in several physical and chemical parameters which would affect seismic attenuation properties in the region of magma intrusion. Reasoning that such changes should be detectable through tomographic analysis, Martinez-Arevalo et al. analyzed 291 shallow seismic events that occurred just before the onset of the 2001 eruption to obtain high-resolution three-dimensional images of the seismic attenuation of P-waves down to a two kilometer [one mile] depth. The tomographic images revealed a very high attenuation volume where the 2001 dike emplaced. The authors interpret this anomaly as the effect of fluid intrusion (magma rich in gas) in the uppermost part of the Etna volcano, making this study the first to yield tomographic images of magma in the process of intruding. Their work confirms that attenuation is a physical parameter, sensitive to the thermal state of crustal volumes through which the seismic waves travel.

Title: The intrusive process leading to the Mt. Etna 2001 flank eruption: Constraints from 3-D attenuation tomography

Authors:
Carmen Martinez-Arevalo and Jesus M. Ibanez: Instituto Andeluz de Geofisica, Universidad de Granada, Granada, Spain;
Domenico Patane: Sezione de Catania, Instituto Nazionale de Geofisica e Vulcanologia, Catania, Italy;
Andreas Reitbrock: Department of Earth and Ocean Sciences, Liverpool University, Liverpool, U.K.

Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL023736, 2005

5. A decrease of emissions is required to stabilize atmospheric CO2

Stabilization of atmospheric carbon dioxide is a key target set out in the 1992 United Nations Framework Convention on Climate Change. Positive feedbacks between the carbon cycle and climate have the potential to accelerate the accumulation of atmospheric carbon dioxide over the next centuries. To study this hypothesis, H. Damon Matthews isolated these feedbacks to estimate the extent to which they must be considered when setting carbon dioxide emission targets. Assuming a carbon dioxide stabilization level at 1000 parts per million by volume, he conducted a coupled climate-carbon cycle simulation consistent with a prescribed carbon dioxide stabilization pathway. Comparing this simulation with a constant-climate simulation, the author shows that carbon cycle-climate feedbacks would require allowable emissions to be reduced significantly, by 94, 230, and 754 gigatons of carbon between now and the years 2050, 2100, and 2350, respectively. His model also shows that while terrestrial feedbacks will dominate this process over next two centuries, oceanic feedbacks will become more significant by the year 2250. Matthews suggests that his estimates can be used to help set emission targets aimed at stabilizing atmospheric carbon dioxide.

Title: Decrease of emissions required to stabilize atmospheric CO2 due to positive carbon cycle-climate feedbacks

Author:
H. Damon Matthews: Department of Geography, University of Calgary, Calgary, Alberta, Canada.

Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL023435, 2005

6. Mapping lava tube structures and depths using ground penetrating radar

The formation of lava tubes is a significant factor controlling the emplacement of lava flows. In contrast to channel-fed or surface lava flows, repeated or continuous use of lava tubes results in an extensive flow field that transports lava over long distances, through a complex network of tubes. However, extents and structures of lava tubes are not precisely known, because field observations of lava tube networks are difficult. Past studies used microgravity and seismic reflection and refraction assessments, but these measurements have vertical resolutions that do not meet the submeter [subyard] scales required for lava tube mapping. To overcome this limitation, Miyamoto et al. developed a new stepped-frequency ground penetrating radar (GPR) system, which yields measurements that have both high spatial resolution and large penetration depth. They studied an inactive lava flow on Mt. Fuji, and found that their GPR system not only easily detected the existence of a known lava tube, but was able to estimate its vertical dimension and its depth, parameters later validated by non-remote field surveys. The authors suggest that this method is a practical approach to mapping terrestrial and perhaps extraterrestrial lava tubes.

Title: Mapping the structure and depth of lava tubes using ground penetrating radar

Authors:
Hideaki Miyamoto, Katsuro Mogi, and Shuichi Rokugawa: Department of Geosystem Engineering, University of Tokyo, Tokyo, Japan;
Jun'ichi Haruyama, Tatsuaki Okada, and Toshiyuki Nishibori: Institute for Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan;
Takao Kobayashi: Japan Science and Technology Agency, Tohoku University, Sendai, Japan;
Keiiti Suzuki: Kawasaki Geological Engineering Co. Ltd., Tokyo, Japan;
Adam P. Showman and Ralph Lorenz: Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA;
David A. Crown: Planetary Science Institute, Tucson, Arizona, USA;
Jose A. P. Rodriguez: Department of Earth and Planetary Sciences, University of Tokyo, Tokyo, Japan;
Tomochika Tokunaga: Institute of Environmental Studies, University of Tokyo, Tokyo, Japan;
Kazuhiko Masumoto: Kajima Corporation, Tokyo, Japan.

Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL024159, 2005

7. Simultaneous interplanetary magnetic field reconnection aurorae in the northern and southern hemispheres

During periods of sustained northward interplanetary magnetic field (IMF) movement and high solar wind pressure, the IMF can reconnect with the Earth's magnetosphere, merging field lines and forming a bright, long-lived, ultraviolet auroral spot, called a cusp aurora. On 18 September 2000, such cusp aurorae were simultaneously viewed for 15 minutes by the Polar and IMAGE satellites in northern and southern hemispheres, respectively. These rare images gave a unique opportunity to examine the IMF and dipole tilt control of the cusp aurora and theta aurora. The latter aurora occurs when the interaction with a northward IMF splits the view of the auroral oval with a transpolar arc, forming an oval that looks like the Greek letter theta. Ostgaard et al. used these images to verify their models of the reconnection geometry and predicted spot locations of such aurorae.

Title: Simultaneous imaging of the reconnection spot in the opposite hemispheres during northward IMF

Authors:
N. Ostgaard: Department of Physics and Technology, University of Bergen, Bergen, Norway;
S. B. Mende and H. U. Frey: Space Sciences Laboratory, University of California, Berkeley, California, USA;
J. B. Sigwarth: Laboratory for Extraterrestrial Physics, Electrodynamics Branch, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL024491, 2005

8. Energetic Neutral Atoms in Saturn's Magnetosphere

Energetic neutral atoms (ENAs) are formed when singly charged ions undergo charge-exchange collisions with neutral gas, with ENAs retaining nearly the same energy as the parent ions. Cassini's Ion and Neutral Camera (INCA) has taken many images of Saturn's inner magnetosphere in ENAs. In some instances, these images were dominated by a periodic signal that persisted for several days. Paranicas et al. were able to obtain a good fit to the data by assuming the signal consisted of a symmetric component and a corotating component. The latter was thought to be an injected population of ions that corotates with the magnetosphere, emitting ENAs along its path. The authors show that the magnetospheric ENA signal strength varies with the distance to the spacecraft, as expected. The images were also strongly influenced by the spacecraft's position with respect to the direction of the magnetospheric plasma flow. This effect, known as the Compton-Getting effect, has been well-documented in ion measurements. This phenomenon was unrecognized in ENA imaging until this study.

Title: Periodic intensity variations in the global ENA images of Saturn

Authors:
C. Paranicas, D. G. Mitchell, E. C. Roelof, P. C. Brandt, D. J. Williams, S. M. Krimigis, and B. H. Mauk: Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL023656, 2005

9. The Canadian Arctic is heavily polluted by lead from Eurasian haze

Eliminating leaded gasoline in Japan, Europe, the U.S., and Canada has had a dramatic effect on atmospheric lead concentrations and deposition rates. Despite this, lead levels are still extremely high relative to the natural background. To compare recent atmospheric lead concentrations with natural values, and to document any trends, Shotyk et al. studied recent snow from Devon Island in the Canadian Arctic, a location remote from all industrial emissions. Noting that the natural ratio of lead to scandium is thought to range between 1 and 5, and that there are no known human sources of scandium, the authors used scandium to approximate background lead levels. Their analysis found a lead-to-scandium ratio averaging 105 for the past ten years. Thus, 95 to 99 percent of this lead was manmade. The isotopic ratios of this lead correlated with Eurasian emissions, confirming previous studies that indicated that the Canadian Arctic is heavily influenced by Eurasian haze. This shows that despite bans on leaded gasoline in some countries, the pollution that persists in other countries continues to heavily contaminate the Arctic.

Title: Predominance of industrial Pb in recent snow (1994-2004) and ice (1842-1996) from Devon Island, Arctic Canada

Authors:
William Shotyk, Jiancheng Zheng, and Michael Krachler: Institute of Environmental Geochemistry, University of Heidelberg, Heidelberg, Germany;
Jiangcheng Zheng, Christian Zdanowicz, Roy Koerner and David Fisher: Geological Survey of Canada, Ottawa, Ontario, Canada.

Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL023860, 2005

10. Pyrite grains yield dates of ancient rocks on Greenland

The Isua greenstone belt in west Greenland is among the most ancient crusts on Earth. In particular, it is the oldest known volcanic-sedimentary sequence. Because these rocks have been deformed, metamorphosed, and metasomatised [changed by reaction with an external source], determining the age of this sequence has been difficult. Past studies have dated the Isua rocks using uranium-lead isotope analyses on zircon crystals, yielding an age of 3.71 to 3.85 billion years. The sedimentary nature of these rocks adds, however, uncertainty to such straightforward dating methods. Instead, Smith et al. applied a single-crystal (40)Argon-(39)Argon dating method to pyrite grains within a minimally deformed banded iron formation from Isua. Their analyses confirm previous determinations of the greenstone's age. In addition, isotopic data from these pyrites indicated that the some of the inclusions may be around 4.31 billion years old, possibly providing the first direct evidence for a much earlier crust at Isua. The authors suggest that given the prevalence of pyrite and other ancient sulfides, their dating method will aid in the search for ancient crust.

Title: Oldest reliable terrestrial (40)Ar-(39)Ar age from pyrite crystals at Isua, west Greenland

Authors:
Patrick E. Smith, Norman M. Evensen, and Derek York: Department of Physics, University of Toronto, Toronto, Ontario, Canada;
Stephen Moorbath: Department of Earth Sciences, Oxford University, Oxford, United Kingdom.

Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL024066, 2005

11. Climate warming is lowering levels of dissolved carbon in the Yukon River

Though climate warming effects on the terrestrial carbon cycle have been extensively studied and modeled at high latitudes, these effects on the transfer of carbon from land to freshwater and marine ecosystems are not as well known, especially for large arctic and subarctic river basins. Past studies suggested that the dissolved organic carbon (DOC) in such basins will increase in response to climate warming, resulting from a predicted increase in DOC release from northern peatlands. However, Striegl et al. found the opposite through their analysis of the Yukon basin. By comparing DOC export, normalized to water discharge, during the growing season from 1978 to 1980 with that taken from 2001 to 2003, they argue that climate warming on frozen soils increases the flow path, residence time, and microbial mineralization of DOC in the soil's active layer and groundwater, ultimately decreasing DOC export. They suggest that similar processes are occurring in other permafrost-dominated river basins, and that continued warming could result in decreased DOC export by major arctic and subarctic rivers, due to increased respiration of organic carbon on land.

Title: A decrease in discharge-normalized DOC export by the Yukon River during summer through autumn

Authors:
Robert G. Striegl: U.S. Geological Survey, Denver Colorado, USA;
George R. Aiken, Mark M. Dornblaser, and Kimberly P. Wickland: U.S. Geological Survey, Boulder, Colorado, USA;
Peter A. Raymond: School of Forestry and Environmental Studies, Yale University, New Haven, Connecticut, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL024413, 2005

12. Internal Tides from Seafloor Seismics

T-waves are seismically generated acoustic waves that propagate through the ocean over long distances along the low velocity Sound Fixing and Ranging (SOFAR) channel, with little loss in the signal intensity. During a submarine volcanic event that occurred north of the Mariana Islands in 1999, T-wave travel times were recorded by Japanese ocean bottom seismometer (OBS) arrays, as well as some of the temporal broadband OBS arrays run by the Ocean Hemisphere Network Project. With these data, Sugioka et al. analyzed the physical oceanography of the region. Using a method of coherent stacking of T-waves, they found that significant variation in T-wave travel times occurred twice every day. At 0.1 to 0.2 seconds, the amplitude of this variation was an order of magnitude larger than those reported in previous ocean sound transmission experiments. The authors find that this difference is due to large internal tides, the vertical motion of water fueled by energy expended during gravitational tides. They suggested that in order to accurately describe oceanic mixing, the role of internal tides at all oceanic locations must be understood.

Title: Submarine volcano, acoustic waves, and internal ocean tides

Authors:
Hiroko Sugioka and Yoshio Fukao: Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan;
Toshiyuki Hibiya: Departments of Earth and Planetary Science, University of Tokyo, Tokyo, Japan.

Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL024001, 2005

II. Ordering information for science writers and general public

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